Recently, in response to distribution of mobile electronic devices such as a mobile phone and a mobile computer, a battery such as a lithium-ion rechargeable battery having a high energy density is being widely distributed as a power supply for mobile electronic devices. In addition, a lithium-ion rechargeable battery is applied to a variety of kinds of use such as an electric power tool, a domestic backup power supply, and an electric vehicle including a small-sized electronic device as well. Accordingly, for a lithium-ion rechargeable battery, there is a strong demand for an additional decrease in size and weight, higher stability, and a longer service life.
A lithium-ion rechargeable battery includes a cathode, an anode, an electrolytic solution, and a separator. As an electrode material constituting the cathode, a lithium-containing metallic oxide having properties capable of reversibly intercalating and deintercalating lithium ions such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium iron phosphate (LiFePO4) is used.
As a charge and discharge reaction of a rechargeable battery proceeds, the internal resistance increases due to electric decomposition of water in the battery or decomposition of an electrolytic solution or the durability or the stability degrades due to generation of gas, and thus the battery characteristics deteriorate. In addition, there is another concern that an electrode material may be dissolved due to hydrofluoric acid generated from a reaction between water and an electrolyte or metal ions eluted in the electrolytic solution may precipitate on the surface of the anode and thus the durability may degrade.
In order to improve the durability or the stability, it is essential to reduce the amount of moisture in the battery. A number of studies are made regarding additives that are added to an electrolytic solution in order to trap moisture, metallic ions, and hydrofluoric acid in the battery. However, there is a concern that battery characteristics may be degraded since a protective film derived from the additives is formed on the surface of an electrode material and thus reaction resistance is increased due to the protective film (for example, refer to Patent Document 1). In order to suppress the above-described disadvantage of the battery, it is necessary to closely investigate additives and decrease the amount of moisture in the electrode material or suppress adsorption of moisture to the electrode material.